10 BRITISH OOLITIC AND LIASIC BRACHIOPODA. 



only to a certain distance, while at other times they dichotomize or unite into strong, 

 wide striae, especially towards and between the fissure and anterior margin. Interior 

 unknown ; length and breadth 19, depth 7 lines. 



Obs. This is the largest and finest species of orbicula with which I am acquainted, 

 and only approached by a specimen shown me by M. D'Orbigny, from the lias of France, 

 believed to be distinct by that author, both species forming part of a sub-genus among 

 the orbiculse proposed by him, under the name of Orbiculoidea. 



I am indebted to the liberality of the Geological Survey for the loan of this magnificent 

 specimen, forming part of their collection, and named by Professor Forbes after the late 

 Mr. Townshend, who found and bequeathed it to that establishment. Unfortunately the 

 ticket has been lost, but I am assured that it is from the Oxford clay beds of the southern 

 districts of England. 



3. Orbicula reflexa, Sow. Plate X, fig. 8. 



Orbicula reflexa, Sow. 1829. M. C, vol. vi, p. 4, pi. 506, fig. 1. 



— — Zool. Journal, vol. ii, p. 321. 



— — Morris. Catalogue, 1843. 



— — Bronn. 1849. Index Palseont., p. 848. 

 Orbiculoidea reflexa, If Orb. 1849. Prodrome, vol. i, p. 258. 



Diagnosis. Shell bivalve, subelliptical, thin, upper or unattached valve convex, with 

 apex directed towards and near the posterior margin ; surface shining and smooth, with 

 the exception of numerous fine concentric lines ; structure horny ; lower or attached valve 

 nearly flat; aperture for the muscular byssus large and elongated. Length 7, breadth 6, 

 depth 3 lines. 



Obs. Two fine specimens of this orbicula are to be seen in the collection of the 

 British Museum, attached to an area, and said to be from the lias of Northampton. We 

 find Peak Whitby mentioned by Mr. Morris, but never having found the shell in situ can 

 add no other details. It has sometimes been mistaken for a similar but more circular 

 orbicula, found in the coal measures of Coalbrook dale, and it seems even probable that 

 Sowerby's figures were drawn from some specimens of that species. 



4. Orbicula Humphresiana, Sow. Plate I, [figs. 3, 3a, 3b. 



Orbicula Humphresiana, Sow. 1829. Vol. vi, p. 5, pi. 506, fig. 2. 



— — Morris. 1843. Catalogue. 



— — Bronn. 1849. Index Palseont., p. 847. 



Diagnosis. Shell bivalve, more or less circular, conical, apex elevated at some distance 

 from the posterior margin ; surface of upper valve ornamented by numerous longitudinal 

 diverging stria3, from the apex towards the marginal line ; lower valve unknown, attached 

 to ostrea deltoidea. Length 6, breadth 5 lines. 



